It takes so very little to winterize an RV, it should be moot to the seller if they really want to make the deal. Insist on them de-winterizing, and YOU watch them do it so you'll know how to do it next year yourself. Test the systems with fresh and pressurized water, via city water hook-up and via water in the fresh water tank and pump. Make sure water heater works on both gas and electric if it has both. YOU can purchase 3 gallons of RV Antifreeze from WallMart and have it ready. After you are satisfied with the water system, have them winterize the camper again, and you watch how how it's done so you can do it again next year too.
Water isn't going to freeze in the camper for the 30 minutes it takes to test all this out. AND you will learn how to winterize and unwinterize, saving yourself a lot of questions next year.
If THEY REALLY want the sale... they will do this. YOU BUY new antifreeze. 3 gallons is probably more than you'll need, but what you don't use, you can keep for next year too. If you buy the new antifreeze, they have NO excuse for not doing this.
IF THEY REFUSE ... run! There is something wrong with the camper they are hiding.